A respiratory disease was first identified in 1976 as being caused by a bacteria that was later named Legionella pneumophila. The disease was named Legionnaires' disease. Since then the disease has occurred in many locations throughout the world including Australia. A recent serious outbreak occurred at the Melbourne Aquarium in 2000 and deaths were reported from this outbreak. In this case cooling towers associated with the facility were identified as the source of the bacteria.
Although previously healthy people may contract the disease, those most at risk are patients undergoing treatment for cancer and other conditions which may render them immunosuppressed. Others at risk include smokers, heavy drinkers and persons with underlying medical conditions such as diabetes and chronic obstructive lung diseases. The mean age of those afflicted has been in the mid-fifties with males being more likely than females.
Transmission is believed to be by inhalation of contaminated aerosols. Infection is not believed to be caused by drinking Legionella contaminated water.
L. pneumophila, one of more than twenty currently recognised species in the genus Legionella, accounts for approximately 75 percent of the cases reported in Australia in the State of Victoria. There are currently fourteen recognised serogroups of L. pneumophila. Infection with other species, namely L. bozemanii and L longbeachae, has also occurred in Victoria. Strictly speaking, Legionnaires' disease is the term used for the disease caused by L. pneumophila, legionellosis refers to the disease caused by any species of Legionella. In this specification the term “Legionnaires' disease” will be used to refer to the pneumonic form of the disease caused by any species of Legionella. 
Legionnaires' disease manifests as severe pneumonia, with patients presenting with early symptoms of malaise, muscle pains, headache and fever with drenching sweats. Within the first week, a cough, usually dry, develops. This occasionally is accompanied with bloodstained sputum. The patients become increasingly short of breath and the respiratory symptoms progress to pneumonia, often culminating in respiratory failure. There is usually a multi-system involvement, with mental confusion and delirium, diarrhoea, vomiting and renal failure. The disease usually has an incubation period of two to ten days. In the treatment of Legionnaires' disease, erythromycin is usually the drug of first choice, with the possible addition of rifampicin in non-responding cases.
Bacteria in the genus Legionella are widely distributed and are natural inhabitants of waters. They have been found in lakes, rivers, creeks and other bodies of water. Other bacteria and algae can provide nutrients for Legionella and may themselves cause health problems if aerosols containing such bacteria or algae are inhaled.
As set out previously, the route of human infection is considered to be by the inhalation of aerosols containing Legionella. Aerosols of five microns diameter or less are particularly effective at penetrating to the lower human respiratory tract. Several of the bacteria in aerosols are reported to be at a maximum at around 65 percent relative humidity. The risk of acquiring infection by Legionella increases with the number of bacteria deeply inhaled and, therefore, with the proliferation of these bacteria in the water source and the extent to which they are dispersed in aerosols of suitable size.
Aerosols may be generated during the normal operation of cooling towers.
Cooling towers are provided in some air conditioning systems to remove the heat rejected by the chiller(s). They are also used for removing heat generated in many industrial processes including refrigeration of air and liquids. In this specification we refer to these collectively as fluids. Typically, water is circulated from a basin via a heat exchanger to the top of the tower where it sprays or falls through a structure which is designed to create an extensive wetted surface area through which air passes. The movement of the air causes evaporative cooling of the water. In this specification the term cooling tower includes cooling towers of all configurations and includes heat rejecting equipment where water is circulated from a reservoir and sprays or falls over heated material. It involves air movement and evaporative cooling of the water.
In this specification the term chiller(s) means all equipment that rejects heat to cooling water and cooling tower systems. In this specification the term cooling water includes condenser water.
In operation, the cooling water gains heat from chillers or other heat exchanger source, and in the process of being distributed over the tower, loses heat to the moving air through evaporative cooling and convective and conductive heat exchange. The mode of airflow is usually forced in commercial systems but by forced and/or convection for industrial plants.
In the operation of all cooling towers, water is lost through evaporation, bleed-off and drift. Drift is the portion of the circulating water entrained in the cooling tower discharge as very small droplets or aerosol. These are produced within the tower by water impacting on the tower fill and also by the water distribution system. The airflow may entrain the smaller droplets and carry them through the tower. To minimise the drift loss, eliminators are frequently located before the tower discharge. Water lost by the above processes is provided by make up water which is usually delivered directly to the cooling tower reservoir or basin. The quantity of make up water is normally one of the order of percent of the flow rate of the recirculating cooling water.
To maximise the effective life of the cooling tower and associated equipment, it has been standard practice for decades to chemically treat the circulating water. Corrosion inhibitors are used to minimise the corrosion of metal surfaces. Surfactants, biocides and other chemicals are used to control fouling due to scale, silt and microbial growths in order to maintain efficient heat transfer at heat transfer surfaces, ensure free flow of water throughout the system, and prevent the proliferation of certain microorganisms which are responsible for surface corrosion and degradation. Biocides must come into contact with the microorganisms to ensure adequate control. Particulate matter, scale, debris, slimes and the presence of other microorganisms such as protozoa have the potential to shield Legionella from biocides and this may result in their persistence and proliferation if biocide levels fall or the Legionella are shielded. Since the increased awareness of Legionnaires' disease potential from cooling towers, there has been an increased use of biocides as a control method.
A major deficiency of the majority of biocides is the lack of a simple on-site test to determine and continuously monitor their concentration in cooling water. Consequently, initial biocidal concentration is commonly determined by calculation based on the estimated water volume of the system and the weight of biocide added. Further disadvantages of the reliance upon biocides are their high cost, handling hazards for operators and adverse environmental effects, particularly in aerosol form and discharges to sewer. Additionally, there is the potential problem that bacteria may develop resistance necessitating the use of new, more potent biocides or combination of biocides. Furthermore, in practice a common reaction to reduce the risk of Legionella is to dose more frequently. The potential hazards from such high levels of chemicals in aerosol form has not be assessed.
The approach to treating cooling towers to date has included compulsory registration, determining existing locations of all cooling towers, development of risk management plans, plus application of compulsory cleaning and chemical treatment standards and regimes.
Another approach has been to avoid the use of cooling towers. Alternative engineering solutions are available that use air cooled chillers or air cooled heat exchangers. However, the operation of these chillers and air cooled heat exchangers involves significant additional costs because of high electricity or other energy usage and demand. The additional energy costs of air cooled chillers may be typically 30% over the energy cost of a cooling tower system. It is desirable for systems to be energy efficient and allow designers to select lowest life cycle cost alternatives. While the capital costs are not greatly different for a new installation, retrofit into existing systems would usually involve substantial additional costs.
A novel approach to disinfecting Legionella is disclosed in WO91/02935. In this patent application a method is described where the coolant water in a collection tank that forms part of a cooling tower is heated to a temperature to kill the unwanted bacteria for a period of four hours each month. Such a system does not appear to have been commercialised and it is considered that such a treatment is not sufficiently efficacious. In the system of this disclosure, one of the shortcomings is that it does not address the problem of bacteria in the heat rejection equipment. The operation of the system also teaches that monthly treatment will control the bacteria levels between treatments. In practice it is believed that bacteria levels can significantly increase over periods of days or weeks. Additionally, the system of this disclosure is only suitable for use where the heat rejecting plant and cooling water system can be shut down during the time of treatment. Increasingly cooling towers are required to be available to support chillers operating on a 24 hour basis.
Another novel approach is described in WO 90/02707. This prior art discloses a building cold water system including cooling tower make up water where the make up water is pasteurised. However, this invention does not address the issue of recirculated water from cooling towers which, as discussed previously, constitutes the vast majority of the water in the system. The make up water normally represents only approximately 1% of the recirculating cooling water. It is the recirculated water that is believed to contribute most to the undesired bacteria. Only a very minor amount of undesired bacteria is believed to be sourced from the inlet water.
It would be desirable to have a method of controlling Legionella in cooling towers without being solely dependent on high levels of biocides.